Different types of capacitors are known, for example film foil capacitors which are inductive capacitors and which are generally manufactured by winding or rolling up a film including dielectric and metal layers, for example a Mylar layer. Another type is a metalized film capacitor which can be made by winding one or more films in several turns around the core, removing the core and cutting out flat sections of the winding, or cutting out curved sections of the winding and forming them into a flat shape, for example by stamping. The cut-up portion has a structure of alternating dielectric layers and metal layers, for example metal foil or metal film layers. On a small portion of the dielectric layer, no metal is film is present, but a so called free margin in formed. This free margin is alternated between the two sides of the capacitor so that by metal spraying the respective opposite side surfaces of the cut out capacitor, two electric terminals can be formed by which the metal layers in the layer structure are contacted alternatingly. In such configuration, two neighboring metal layers are separated by a dielectric layer and form a capacitor, so that the cut-out capacitor essentially corresponds to a parallel connection of plural capacitances. Such cut out capacitor with its metalized sides can be termed capacitor stack.
When attempting to deploy such metalized film capacitor in a high pressure environment, for example inside a pressure compensated subsea enclosure which is installed at the ocean floor, several problems are encountered. In such enclosure, the metalized film capacitor is exposed to the ambient pressure, which at an installation depth of up to 3.000 meters and more results in a pressure of 300 bar or more prevailing in the environment in which the capacitor is installed. Gas that is trapped between the layers of the capacitor may damage the capacitor upon application of such high pressures. Furthermore, the layers may delaminate in such high pressure environment. The layers of the capacitor may experience buckling or wrinkling, which generally leads to a failure of the capacitor.
Furthermore, it was found that such capacitor stack may not have a capacitance that is high enough for the intended use, for example in the DC link of a subsea variable speed drive which provides AC frequency conversion for driving a subsea load, such as the electric motor of a subsea pump, subsea compressor or the like.
The document GB 500 167 A discloses a ceramic high tension electric condenser that include a column of superimposed ceramic plates provided with metal layers constituting the condenser plates. Springs are interposed between the plates to obtain good conductive connection between adjacent metal layers.
The document US 2014/0087231 A1 discloses an energy storage apparatus in form of a battery having plural galvanic cells, such as lithium ion cells which are pressed against each other by means of two clamping straps.
The document FR 1 581 052 A discloses a condenser with plural flat condenser coils that are separated by insulating plates.